The present invention relates to a technique for diagnosing a response characteristic of an air-fuel ratio sensor, while feedback controlling an air-fuel ratio based on a detection result of the air-fuel ratio sensor.
Japanese Unexamined Patent Publication No. 11-264340 discloses system in which a target air-fuel ratio is forcibly changed during an air-fuel ratio feedback control. Based on a change in output of an air-fuel ratio sensor to the change in the target air-fuel ratio, a response deterioration of the air-fuel ratio sensor is diagnosed. In this conventional diagnosis process, as the target air-fuel ratio is forcibly changed and as a feedback gain is increased during the diagnosis, the target air-fuel ratio may greatly deviate from a primary target air-fuel ratio at the time when the diagnosis is finished.
When the diagnosis is finished, however the feedback gain is returned to a normal feedback gain and, therefore, it takes time to return the target air-fuel ratio to the primary target air-fuel ratio, thereby resulting in the deterioration in exhaust performance during this time. If the feedback gain is maintained high even immediately after the diagnosis is finished, although the target air-fuel ratio can be returned rapidly up to the vicinity of the primary target air-fuel ratio, an errant overshoot of air-fuel ratio may be caused.